Method for calibrating an electronically phase-controlled group antenna in radio communications systems

ABSTRACT

An electronically phase-controlled group antenna is calibrated in radio communications systems, using a reference point shared by all the reference signals. In the downlink, reference signals which can be distinguished from one another are simultaneously transmitted by individual antenna elements of the group antenna and are suitably separated after reception at the shared reference point.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to PCTapplication Ser. No. PCT/DE00/03756 filed on Oct. 24, 2000 and GermanApplication No. 199 51 525.5 filed Oct. 25, 1999, the contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates a method for calibrating an electronicallyphase-controlled group antenna, using a reference point shared by allthe reference signals, in radio communications systems and to anarrangement for this.

By the use of electronically phase-controlled group antennas, known asintelligent antennas, in radio communications systems, such as forexample digital mobile radio systems, a directional selectivity of amobile radio channel that exists in spite of multipath propagation canbe advantageously used for the radio communication.

Intelligent antennas form a radiation pattern by correspondingphase-directed activation of the individual antenna elements of theantenna array. The beam forming can therefore be used to transmit amessage from a base station to a subscriber station specifically in thedirection of the latter. As a result, on the one hand the sensitivity tointerferences in the particular radio cell of the base station can bereduced and on the other hand co-channel interferences in neighboringradio cells can be reduced. Moreover, the range of a base station whichis providing a specific mobile station with radio resources increasessignificantly for the same transmit power. In addition, as a consequenceof the spatial separation, physical channels within a radio cell servedby a base station can be reused and the antenna lobes, as they areknown, of the directional diagram can be adaptively corrected whensubscriber stations move.

To achieve a desired beam formation, the original transmission signal issent via a plurality of antenna elements, usually with different, butdefined phase angles. The corresponding phase angle is ascertained foreach antenna element by a digital signal processing (DSP).

Unforeseeable phase errors and time delays generally occur when settingthe phase angle in the analog area between digital/analog converters andantenna elements. As a result, the transmission signals are not sentwith the desired phase angles and the beam formation is falsified oreven impossible. To counteract this unfavorable property of the analogarea of beam formation, what is known as antenna calibration isnecessary. Antenna calibration eliminates the influence of the entireanalog signal chain on the errors described above.

To use beam formation, firstly the direction of the base station inrelation to the mobile station must be established. The direction isestablished by evaluation of the various phase angles of the receivedsignal at each antenna element of the antenna array. Therefore, anantenna calibration in the base station is necessary not only for thedownlink to the subscriber station but also for the uplink from thesubscriber station to the base station.

In a TD-SCDMA system (Time Division-Synchronous Code Division MultipleAccess System), using intelligent antennas, an additional antenna, knownas a reference antenna, is used for the antenna calibration. For thecase of an uplink calibration, a reference signal is sent via thereference antenna to all the antenna elements of the antenna array. Atthe individual antenna elements, a specific delay time and a specificphase position, depending on the distance from the reference antenna,are expected on account of the finite propagation velocity ofelectromagnetic waves. The difference between the expected setpointvalue and the actually measured actual value is ascertained and storedas a correction factor. The correction factor is then included in thenormal signal processing process, whereby the antenna is calibrated.

For the downlink calibration, the reference antenna receives at aspecific point in time a reference signal from an antenna element of theantenna array, and the correction factor is determined. To counteractthe distortion of the measurement results on account of differentantenna elements of the antenna array, they must not transmit a signalat this point in time. Subsequently, at a second point in time, thereference antenna receives a reference signal from a second antennaelement of the antenna array, and the correction factor for this secondantenna element is determined, and so on. For the calibration of nantenna elements of the antenna array, accordingly n time slots must beused when supporting a TDMA subscriber separation method (Time DivisionMultiple Access).

The error in the delay time is often only a fraction of a chip(chip=CDMA code element). To take such a small delay time into accountin the signal processing, an oversampling of the received signal andtransmission signal is necessary. However, oversampling makes the datarates to be transmitted considerably greater.

SUMMARY OF THE INVENTION

The invention is based on the object of significantly shortening thetime for the calibration of intelligent antennas in the downlink.

A further object is to perform a correction of the analog error withoutthe necessity of calculating a correction factor for each antennaelement and without oversampling and the associated higher data rates.

A further object is to keep down the load on the transmission capacityof physical channels caused by an antenna calibration.

According to one aspect of the invention, all the antenna elements of anintelligent antenna in the downlink are calibrated in only one step. Forthis purpose, reference signals which can be distinguished from oneanother are simultaneously sent by the individual antenna elements ofthe antenna array and are separated again after reception at a referencepoint shared by all the reference signals.

An advantageous refinement provides a separation of the referencesignals using a CDMA method (CDMA=Code Division Multiple Access), whichis based on a separation of signals by individual spread-spectrum codes.

In a further refinement, conventional spread-spectrum code techniques,such as correlation, in which the common reference point is synchronizedto the respective reference code channel of the antenna elements and thereference signals are again reduced to their original bandwidth, areused for the separation of the reference signals.

According to a further refinement, in this case the reference signalsare orthogonally coded, in order that the interferences remain minimalin spite of simultaneous transmission.

The calibration factor can be obtained from the result of thecorrelation in a digital signal processor.

Another advantageous form of the invention is to use an optimized amountof reference signals, which allows an impartial estimate of thecalibration factor.

The generation of such an optimized amount of reference signals and ofthe estimated value can be performed in an advantageous way by methodswhich are described in: Bernd Steiner, Paul Walter Baier: “Low Costchannel Estimation in the uplink receiver of CDMA mobile radio systems”,Frequenz 47 (1983), pages 292-298.

According to a further form, the correction of the delay time, phaseerror and/or amplitude of the transmission signals can be performeddirectly within a digital UP-conversion/down-conversion, whereby nocorrection factor has to be included and no oversampling of the receivedsignal and transmission signal is necessary to eliminate delay errors.

For this purpose, tuning of the numerically controled oscillator (NCO)of the digital UP-converter (DUC) and of the digital down-converter(DDC) takes place.

In a further development, in a TDD system the calibration is carried outin the delay time without transmission between the uplink and downlinktime slots.

In a further refinement, the downlink calibration may take place at thebeginning of the delay time and the uplink calibration may take place atthe end of the delay time.

In a further refinement, a reference antenna is used as the sharedreference point for the reference signals from and to the antennaelements.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 schematically shows a radio communications system usingintelligent antennas,

FIG. 2 schematically shows the signal flow in an uplink synchronizationof an intelligent antenna to be calibrated,

FIG. 3 schematically shows the signal flow in a downlink synchronizationof an intelligent antenna to be calibrated, and

FIG. 4 schematically shows the signaling for an antenna calibration in adelay interval between the uplink and downlink in the TTD mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

FIG. 1 shows a base station BS, which has, by way of example,established communication in the area of its served radio cell Z withthree mobile stations MS. For an undisturbed connection from and to themobile stations MS, a channel separation by a time division duplexmethod TDD is provided. For the separation of the connections betweenthe individual mobile stations MS, the hybrid multiple access methodTD-SCDMA (Time Division-Synchron Code Division Multiple Access), a formof TD-CDMA (Time Division-Code Division Multiple Access) may be used byway of example. TD-CDMA is a combination of the multiple accesscomponents TDMA (Time Division Multiple Access) and CDMA (Code DivisionMultiple Access) and is characterized by the degrees of freedomfrequency, time slot and code. TD-SCDMA differs from TD-CDMA by the useof a highly accurate synchronization of the received signals in theuplink. As a result, the orthogonality of the received signals isretained to the greatest extent, resulting in turn in an improvement inthe detection properties.

A precondition for a TD-SCDMA system or a comparable radiocommunications system with intelligent antennas is to have antennas withwhich a directional selectivity of the transmission signals transmittedfrom a base station BS can be achieved. With intelligent antennas,electronically steerable, highly focusing propagation diagrams can beproduced. Consequently, intelligent antennas reduce the angles ofincidence for detours caused by the surroundings in the path of thetransmission signals to the mobile stations, whereby the interference isreduced. From the same base station BS, it is consequently possible fordifferent antenna lobes, which are steered in different directions, touse simultaneously the same frequency channel within a cell Z. What ismore, the range of a base station BS increases for the same transmitpower.

In FIG. 1, the intelligent antenna of the base station BS detects thedirections from which the mobile stations MS are sending and formscorresponding antenna lobes in their direction.

Schematically represented in FIG. 2 is the signal flow in the case of anuplink calibration of an intelligent group antenna, comprising aplurality of antenna elements AE1 to AEN and a reference antenna AR forthe calibration. The arrows illustrate the different transit time of areference signals from a reference antenna AR to the antenna elementsAE1 to AEN. The reference signals picked up by each antenna element AE1to AEN, and amplified if need be, are digitized parallel to one anotherin analog/digital converters A/D. The digitized values are subsequentlyhandled in parallel in a digital down-converter DDC. From the measuringsignals obtained in this way, it is possible for example to ascertaincorrection factors in a digital signal processor DSP and return thecorrection values as control information to the digital down-converterDDC of the individual antenna elements AE1 to AEN. What is more, thereference signals from the signal processor DSP are sent via a digitalup-converter DUC and a digital/analog converter D/A to the referenceantenna AR, which sends said signals to the antenna elements AE1 to AENfor the purpose of calibration, etc.

Schematically represented in FIG. 3 is the signal flow in the case of adownlink calibration of an intelligent group antenna. The antennaelements AE1 to AEN each send a reference signal simultaneously to thereference antenna AR, which receives said signals with differentreference signal transit times. If need be, the reference antenna ARamplifies the reference signals and converts them back into digitalsignals in an analog/digital converter A/D. The digitized signals aresubsequently handled in a digital down-converter DDC and the measuringsignals obtained in this way are fed to the digital signal processorDSP. In the signal processor DSP, correction factors are ascertained,for example, from the measurement results and passed as controlinformation to the digital UP-converters DUC of the antenna elements AE1to AEN. What is more, reference signals 1 to N are fed to the digitalUP-converters DUC for the purpose of transmission by the antennaelements AE1 to AEN.

Selected below is a computational example for a TD-SCDMA system using anintelligent antenna with 8 antenna elements, a reference antenna and alength of the CDMA code elements (chip) of 0.75 μs.

The determination of the calibration factor takes place in a wayanalogous to channel-estimating methods known from mobile radiotechnology. The time delay and the phase position of the receivedreference signals are determined. Since the delay error is very small incomparison with the setpoint delay value, three measurements of channelpulse responses for each antenna element in the time available areadequate for example. Consequently, the signal length for thecalibration of all the antenna elements of an intelligent antenna in thedownlink is: (8+1) antenna elements*3 measurements*0.75 μs chiplength=20.25 μs.

The antenna calibration, that is to say the correction of the influenceof the analog error over the entire signal chain on the directionalpattern of the intelligent group antenna, is carried out directlydigitally. No oversampling of the received signal and transmissionsignal is necessary to eliminate delay errors.

In modern base stations, digital UP-conversion and down-conversion isused to compensate for problems caused by IQ phase errors and IQamplitudes offsets. The correction of the delay time and phase of thetransmission signals can be achieved directly by tuning the numericallycontroled oscillator NCO (Numarical Controled Oscillators) of thedigital UP-converter (DUC) and of the digital down-converter (DDC),without a correction factor having to be included in the digital signalprocessing in the DSP.

Digital up-converters DUC and digital down-converters DDC also permittuning of the amplitude of the transmission signals, since anerror-affected amplitude likewise influences the beam formation.

On account of the high data rates between the calibration instance andDUC/DDC, the disadvantage of additionally signaling control informationto DUC and DDC is negligible.

It can be seen from FIG. 4 that, in a TDD system, such as for exampleTD-SCDMA, a delay time of a certain length is provided between theuplink and downlink for counteracting transit time differences of thesignals and data to be transmitted. The calibration measurementspreferably take place in this delay time, since at this point in time nofurther signals can influence the measurements. The downlink calibrationis preferably carried out at the beginning of the delay time and theuplink calibration is preferably carried out at the end of this time.

In the same way, a time slot TS provided for communication connectionscan also be reserved for the calibration procedure described.

The frequency of the antenna calibration is freely selectable and can beadapted dynamically to the transmission requirements. For example, acalibration may be performed in the downlink and uplink in each delaytime between downlink and uplink TDMA frames or else a calibration isperformed with a time interval which is a multiple thereof. Thefrequency of a downlink calibration may also differ from the frequencyof an uplink calibration, for example if it is established by the basestation that a mobile station is moving only insignificantly or not atall during a communication connection, for example for voicetransmission, for data transport or for multimedia transmission.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

What is claimed is:
 1. A method for calibrating an electronicallyphase-controlled group antenna having n antenna elements in a radiocommunications system, comprising: in the case of an antenna calibrationin the upward direction, transmitting a reference signal from a sharedreference antenna to each of the n antenna elements, the referencesignal having a different transit time to the n antenna elements; in thecase of an antenna calibration in the upward direction, passing ameasuring signal from each of the n antenna elements to a device forerror correction; in the case of an antenna calibration in the upwarddirection, ascertaining an error correction value for each of the nantenna elements in relation to a reference point, the error correctionvalues being determined at the device for error correction based oncorresponding measuring signals; in the case of an antenna calibrationin the downward direction, transmitting a reference signal from each ofthe n antenna elements to the reference antenna, such that each antennaelement has an assigned reference signal and the n antenna elements formn reference signals at the same time; in the case of an antennacalibration in the downward direction, receiving the reference signalsin superposed form at the reference antenna, each of the referencesignals having a different transit time; in the case of an antennacalibration in the downward direction, passing a shared measuring signalfrom the reference antenna to the device for error correction, theshared measuring signal carrying information regarding the transit timeof each of the reference signals; and in the case of an antennacalibration in the downward direction, ascertaining from the sharedmeasuring signal an error correction value for each of the n antennaelements in relation to the reference point.
 2. The method as claimed inclaim 1, wherein the reference signals are coded and decoded on thebasis of a CDMA method.
 3. The method as claimed in claim 2, wherein acorrelation method is used for the synchronization of the referencepoint to a reference code channel of the antenna elements.
 4. The methodas claimed in claim 3, wherein the reference signals are orthogonallycoded.
 5. The method as claimed in claim 4, wherein a correction of ananalog error in at least one of time delay, phase and amplitude isperformed digitally.
 6. The method as claimed in claim 5, wherein thecorrection is performed within a digital up-conversion or a digitaldown-conversion.
 7. The method as claimed in claim 6, wherein acalibration factor to correct the analog error is obtained from acorrelation in a digital signal processor.
 8. The method as claimed inclaim 7, wherein an optimized amount of signals is used to estimate thecalibration factor.
 9. The method as claimed in claim 8, wherein, in thecase of time division duplex operation, the calibration of theelectronically phase controlled group antenna is carried out within adelay time between the upward direction and the downward direction. 10.The method as claimed in claim 9, wherein the reference signals for thecalibration in the downward direction are sent at the beginning of thedelay time.
 11. The method as claimed in claim 10, wherein the referencesignals for the calibration in the upward direction are sent at the endof the delay time.
 12. The method as claimed in claim 8, wherein thereference signals for calibration in at least one of the upwarddirection and the downward direction are sent in a designated time slot.13. The method as claimed in claim 1, wherein the reference signals areorthogonally coded.
 14. The method as claimed in claim 1, wherein acorrection of an analog error in at least one of time delay, phase andamplitude is performed digitally.
 15. The method as claimed in claim 14,wherein the correction is performed within a digital up-conversion or adigital down-conversion.
 16. The method as claimed in claim 14, whereina calibration factor to correct the analog error is obtained from acorrelation in a digital signal processor.
 17. The method as claimed inclaim 16, wherein an optimized amount of signals is use to estimate thecalibration factor.
 18. The method as claimed in claim 1, wherein, inthe case of time division duplex operation, the calibration of theelectronically phase controlled group antenna is carried out within adelay time between the upward direction and the downward direction. 19.The method as claimed in claim 18, wherein the reference signals for thecalibration in the downward direction are sent at the beginning of thedelay time.
 20. The method as claimed in claim 18, wherein the referencesignals for the calibration in the upward direction are sent at the endof the delay time.
 21. The method as claimed in claim 1, wherein thereference signals for calibration in at least one of the upwarddirection and the downward direction are sent in a designated time slot.